I've used fdisk /mbr before to remove the Linux bootloader. Check the latter part of this link:
http://support.microsoft.com/kb/q171611/
about removing Lilo. It also works for GRUB. In my response, I forgot that the drive may be SATA. In that case, the Windows 98 boot floppy may not work. He said it was an old desktop, so I thought it probably had PATA hard drive.